Chandrayaan-I clears all tests for Moon odyssey

Srinivas Laxman, TNNOct 9, 2008, 11.14am IST

MUMBAI: As the moon mission began progressing from the realms of imagination to reality, scientists put their heads together to design the perfect spacecraft to carry India's soaring aspirations. After slogging it out for months, straddling numerous calculations and combinations, what emerged from the drawing boards was a cuboid spacecraft. It had a single solar array to generate power.

As the spacecraft took shape, the scientists monitored its progress on computers. The equipment was housed in a closely-guarded room at the Isro satellite centre in Bangalore. During launch, the spacecraft will weigh 1,304 kg. Once it reaches the lunar orbit covering a distance of nearly 3,86,000km from the Earth, its weight will go down to 590kg as most of its propellants will be depleted during the five-and-a-half-day journey.

Of its total weight, the 11 scientific payloads it'll carry will weigh 80kg. Initially, when the design of the spacecraft was conceived, the weight of the payloads was kept at 35kg. One of the reasons why it shot up was the inclusion of the 29kg Moon Impact Probe, an Indian payload which was initiated at the instance of former President A P J Abdul Kalam. The probe will crashland on the lunar surface to identify future landing sites and explore the moon from a close range.

After freezing the design, scientists got down to preparing it and, by September, it was in flight mode. But takeoff was still a far cry as the spacecraft had to undergo several crucial tests to assess if it could weather the hostile space environment while encircling the moon for two years.

Chandrayaan-I would take 117 minutes for every revolution at an altitude of 100km above the lunar surface. In the thermovac test, the spacecraft was lowered into a chamber with temperature extremes. "This test imulates the actual difficult space environment," said an Isro official. Next came the vibration test in which the spacecraft was placed on a table and vibrated just like an actual launch.

Third in line was the acoustic test where the roar of four jet planes was simulated simultaneously to check if the spacecraft could survive the literal blast of the launch. Isro officials recalled a case a few years ago when a few cows, which had accidentally strayed into the launch zone, had gone deaf because of the sound and fury of the takeoff.

The lunar vehicle scored full marks in all these tests and is now ready for launch in the early hours of October 22. After takeoff, once it detaches from the rocket and enters the geo-stationary transfer orbit, the spacecraft will operate on its own for a few days.

The spacecraft will generate power through what is known as a canted single-sided solar array. Which means Chandrayaan's energy will be gathered from the sun. The solar array with light-sensitive cells will generate 700 watts during peak period. During eclipse, it will be powered by lithium batteries.

Chandrayaan is equipped with things like thrusters, which will execute fuel burns to alter the trajectory whenever needed. It has an inertial measurement unit consisting of accelometers and gyroscopes to measure the attitude. The design of the spacecraft is so advanced that flight controllers on Earth can instruct it to bring a back-up on line if there is a technical hitch.

It has three solid state recorders on board to record data from various payloads. While SSR-I will store science payload data, SSR-II will carry spacecraft altitude information, satellite housekeeping and other auxilliary data besides science payload data.